Roll-To-Roll Electroplating for Photovoltaic Film Manufacturing

ABSTRACT

A roll to roll system for forming an absorber structure for solar cells on a flexible foil as the flexible foil is advanced through units of the system and by unwrapping from a supply spool and wrapping around a take-up spool. Surface of the flexible foil is first conditioned in a conditioning unit to form an activated surface. A precursor stack including copper, gallium and indium layers is electroplated onto the activated surface by utilizing separate electroplating units for each layers. The precursor layer is reacted with at least one of Se and S in an annealing unit of the system.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to Provisional Application Ser. No.60/862,164 filed on Oct. 19, 2006.

FIELD OF THE INVENTION

The present invention relates to methods and apparatus for preparingthin films of Group IBIIIAVIA compound semiconductor films for radiationdetector and photovoltaic applications.

BACKGROUND

Solar cells are photovoltaic devices that convert sunlight directly intoelectrical power. The most common solar cell material is silicon, whichis in the form of single or polycrystalline wafers. However, the cost ofelectricity generated using silicon-based solar cells is higher than thecost of electricity generated by the more traditional methods.Therefore, since early 1970's there has been an effort to reduce cost ofsolar cells for terrestrial use. One way of reducing the cost of solarcells is to develop low-cost thin film growth techniques that candeposit solar-cell-quality absorber materials on large area substratesand to fabricate these devices using high-throughput, low-cost methods.

Group IBIIIAVIA compound semiconductors comprising some of the Group IB(Cu, Ag, Au), Group IIIA (B, Al, Ga, In, Tl) and Group VIA (O, S, Se,Te, Po) materials or elements of the periodic table are excellentabsorber materials for thin film solar cell structures. Especially,compounds of Cu, In, Ga, Se and S which are generally referred to asCIGS(S), or Cu(In,Ga)(S,Se)₂ or CuIn_(1-x)Ga_(x) (S_(y)Se_(1-y))_(k),where 0≦x≦1, 0≦y≦1 and k is approximately 2, have already been employedin solar cell structures that yielded conversion efficienciesapproaching 20%. Absorbers containing Group IIIA element Al and/or GroupVIA element Te also showed promise. Therefore, in summary, compoundscontaining: i) Cu from Group IB, ii) at least one of In, Ga, and Al fromGroup IIIA, and iii) at least one of S, Se, and Te from Group VIA, areof great interest for solar cell applications.

The structure of a conventional Group IBIIIAVIA compound photovoltaiccell such as a Cu(In,Ga,Al)(S,Se,Te)₂ thin film solar cell is shown inFIG. 1. The device 10 is fabricated on a substrate 11, such as a sheetof glass, a sheet of metal (such as aluminum or stainless steel), aninsulating foil or web, or a conductive foil or web. The absorber film12, which comprises a material in the family of Cu(In,Ga,Al)(S,Se,Te)₂,is grown over a conductive layer 13, which is previously deposited onthe substrate 11 and which acts as the electrical contact to the device.Various conductive layers comprising Mo, Ta, W, Ti, and stainless steeletc. have been used in the solar cell structure of FIG. 1. If thesubstrate itself is a properly selected conductive material, it ispossible not to use a conductive layer 13, since the substrate 11 maythen be used as the ohmic contact to the device. After the absorber film12 is grown, a transparent layer 14 such as a CdS, ZnO or CdS/ZnO stackis formed on the absorber film. Radiation 15 enters the device throughthe transparent layer 14. Metallic grids (not shown) may also bedeposited over the transparent layer 14 to reduce the effective seriesresistance of the device. It should be noted that the structure of FIG.1 may also be inverted if substrate is transparent. In that case lightenters the device from the substrate side of the solar cell.

In a thin film solar cell employing a Group IBIIIAVIA compound absorber,the cell efficiency is a strong function of the molar ratio of IB/IIIA.If there are more than one Group IIIA materials in the composition, therelative amounts or molar ratios of these IIIA elements also affect theproperties. For a Cu(In,Ga)(S,Se)₂ absorber layer, for example, theefficiency of the device is a function of the molar ratio of Cu/(In+Ga).Furthermore, some of the important parameters of the cell, such as itsopen circuit voltage, short circuit current and fill factor vary withthe molar ratio of the IIIA elements, i.e. the Ga/(Ga+In) molar ratio.In general, for good device performance Cu/(In+Ga) molar ratio is keptat around or below 10.0. As the Ga/(Ga+In) molar ratio increases, on theother hand, the optical bandgap of the absorber layer increases andtherefore the open circuit voltage of the solar cell increases while theshort circuit current typically may decrease. It is important for a thinfilm deposition process to have the capability of controlling both themolar ratio of IB/IIIA, and the molar ratios of the Group IIIAcomponents in the composition. It should be noted that although thechemical formula is often written as Cu(In,Ga)(S,Se)₂, a more accurateformula for the compound is Cu(In,Ga)(S,Se)_(k), where k is typicallyclose to 2 but may not be exactly 2. For simplicity we will continue touse the value of k as 2. It should be further noted that the notation“Cu(X,Y)” in the chemical formula means all chemical compositions of Xand Y from (X=0% and Y=100%) to (X=100% and Y=0%). For example,Cu(In,Ga) means all compositions from CuIn to CuGa. Similarly,Cu(In,Ga)(S,Se)₂ means the whole family of compounds with Ga/(Ga+In)molar ratio varying from 0 to 1, and Se/(Se+S) molar ratio varying from0 to 1.

The first technique used to grow Cu(In,Ga)Se₂ layers was theco-evaporation approach which involves evaporation of Cu, In, Ga and Sefrom separate evaporation boats onto a heated substrate, as thedeposition rate of each component is carefully monitored and controlled.

Another technique for growing Cu(In,Ga)(S,Se)₂ type compound thin filmsfor solar cell applications is a two-stage process where at least two ofthe components of the Cu(In,Ga)(S,Se)₂ material are first deposited ontoa substrate, and then reacted with S and/or Se in a high temperatureannealing process. For example, for CuInSe₂ growth, thin sub-layers ofCu and In are first deposited on a substrate to form a precursor layerand then this stacked precursor layer is reacted with Se at elevatedtemperature. If the reaction atmosphere contains sulfur, then aCuIn(S,Se)₂ layer can be grown. Addition of Ga in the precursor layer,i.e. use of a Cu/In/Ga stacked film precursor, allows the growth of aCu(In,Ga)(S,Se)₂ absorber. Other prior-art techniques include depositionof Cu—Se/In—Se, Cu—Se/Ga—Se, or Cu—Se/In—Se/Ga—Se stacks and theirreaction to form the compound. Mixed precursor stacks comprisingcompound and elemental sub-layers, such as a Cu/In—Se stack or aCu/In—Se/Ga—Se stack, have also been used, where In—Se and Ga—Serepresent selenides of In and Ga, respectively.

Sputtering and evaporation techniques have been used in prior artapproaches to deposit the sub-layers containing the Group IB and GroupIIIA components of metallic precursor stacks. In the case of CuInSe₂growth, for example, Cu and In sub-layers were sequentiallysputter-deposited from Cu and In targets on a substrate and then thestacked precursor film thus obtained was heated in the presence of gascontaining Se at elevated temperatures as described in U.S. Pat. No.4,798,660. More recently U.S. Pat. No. 6,048,442 disclosed a methodcomprising sputter-depositing a stacked precursor film comprising aCu—Ga alloy sub-layer and an In sub-layer to form a Cu—Ga/In stack on ametallic back electrode and then reacting this precursor stack film withone of Se and S to form the compound absorber layer. U.S. Pat. No.6,092,669 described sputtering-based equipment and method for producingsuch absorber layers.

One prior art method described in U.S. Pat. No. 4,581,108 utilizes anelectrodeposition approach for metallic precursor preparation. In thismethod a Cu sub-layer is first electrodeposited on a substrate. This isthen followed by electrodeposition of an In sub-layer and heating of thedeposited Cu/In precursor stack in a reactive atmosphere containing Se.This technique was found to require very high plating current densitiesresulting in non-uniformities and problems of adhesion to the substrateas discussed in reference publications (Kapur et al., “Low Cost ThinFilm Chalcopyrite Solar Cells, Proceedings of 18^(th) IEEE PhotovoltaicSpecialists Conf., 1985, p. 1429; “Low Cost Methods for the Productionof Semiconductor Films for CIS/CdS Solar Cells”, Solar Cells, vol. 21,p. 65, 1987).

As the brief review above demonstrates there is still a need to develophigh-throughput, low cost techniques to manufacture thin film solarcells and modules.

SUMMARY OF THE INVENTION

The present invention provides a roll to roll system to form solar cellabsorbers by continuously processing a surface of a flexible foil as theflexible foil is advanced through processing units of the roll to rollsystem.

An aspect of the present invention provides a system for forming anabsorber structure for solar cells on a front surface of a continuousflexible workpiece as the continuous flexible workpiece is advancedthrough units of the system. The system includes a conditioning unit tocondition the front surface of the continuous flexible workpiece to formactivated surface portions.

The system further includes a first electroplating unit to form a firstlayer of a precursor stack by electroplating a metal belonging to one ofGroup IB and Group IIIA of the periodic table on an activated surfaceportion of the continuous flexible workpiece as the continuous flexibleworkpiece is advanced through the first electroplating station. A firstcleaning unit of the system is to clean the first layer deposited in thefirst electroplating unit.

The system further includes a second electroplating unit to form asecond layer of the precursor stack by electroplating a metal belongingto one of Group IB and Group IIIA of the periodic table onto the firstlayer as the continuous flexible foil is advanced through the first andthe second electroplating units and while the first layer is continuedto be electroplated onto a following activated surface portion of thesurface of the continuous flexible foil in the first electroplatingunit. The first layer is different from the second layer. A secondcleaning unit of the system is to clean the second layer deposited inthe second electroplating unit.

The system further includes a third electroplating unit to form a thirdlayer by electroplating a metal belonging to one of Group IB and GroupIIIA of the periodic table onto the second layer to complete theprecursor stack as the flexible foil is advanced through the first,second and third electroplating stations and while the second layer iscontinued to be electroplated in the second electroplating station onthe first layer that is electroplated on the following activated portionof the surface of the flexible foil, and while the first layer iscontinued to be electroplated onto another following activated portionof the surface of the flexible foil in the first electroplating station.The third layer is different from the first and second layers. Thesystem further includes a moving assembly to hold and linearly move thecontinuous flexible workpiece through the units of the system, whereinthe moving assembly comprises a feed spool to unwrap and feedunprocessed portions of the continuous flexible workpiece into thesystem and a take-up spool to receive the processed portions and wrapthem around.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a solar cell employing a GroupIBIIIAVIA absorber layer.

FIG. 2 shows a roll to roll electrodeposition system of the presentinvention.

FIG. 3 shows another roll to roll electrodeposition system of thepresent invention comprising multiple electroplating units and cleaningunits.

FIG. 3A shows a structure of the flexible foil base.

FIG. 4 shows a roll to roll processing system comprising additionalprocessing units including a Group VIA material electroplating unit.

FIG. 5 shows a flow chart of an embodiment of a process using roll toroll system

DETAILED DESCRIPTION

Present invention provides a low-cost, high throughput two-stage processfor fabrication of CIGS(S) type absorber layers for manufacturing ofsolar cells.

FIG. 2 schematically shows an embodiment of the process and the tool ofthe present invention. In this embodiment, a roll-to-roll processingtechnique is used to electrodeposit a Group IB material (preferably Cu)and a Group IIIA material (preferably at least one of In and Ga) in acontinuous manner on a continuous flexible workpiece 22 such as aflexible foil base including a flexible substrate and a contact layer.The tool 19 has a supply spool 20 and a return spool 21 and the flexiblefoil base 22 is directed from the supply spool 20 to the return spool 21through a series of electroplating units 23. The process units 23 mayinclude at least one Group IB material electroplating unit, and at leastone Group IIIA material electroplating unit. After each electroplatingunit 23, there may preferably be cleaning units 24A, 24B. The cleaningunits rinse the electroplated surface after each electroplating process,and thus avoid cross contamination of the electroplating electrolytes orbaths in the electroplating units 23. For example, after a section ofthe base 22 is electroplated or electrocoated with Cu in anelectroplating unit, the section passes through a cleaning unit wherethe chemical residues of the Cu plating bath on the section are rinsedoff and the section moves into a Group IIIA electroplating unit such asa Ga electroplating unit. It should be noted that the section may alsobe dried after the rinsing step; however, in general it is preferable tokeep the surface of the already plated material layer wet as it goesinto another electroplating bath. There is a need to provide a rinse/dryunit 25 at the end of the tool 19 to assure that the flexible foil base22 comprising electroplated Group IB and Group IIIA materials iscompletely clean and dry before being rolled onto the return spool 21.To avoid damage to the electroplated layers, a packing sheet 26 may befed from a packing spool 27, to between the layers of the flexible foilbase 22 comprising the electroplated Group IB and Group IIIA materialson the return spool 21. The packing sheet 26 may be a paper or thinpolymeric sheet.

Flow chart 100 shown in FIG. 5 provides an exemplary process flow for anembodiment of the roll to roll system of the present invention.Initially, as shown in box 101, a contact layer may be formed on thecontinuous flexible substrate to form a continuous flexible workpiece onwhich a precursor stack of the present invention would be built usingthe system of this invention. Next, as shown in box 102, in a surfaceactivation step, surface of the contact layer is conditioned to form anactivated surface for the following electrodeposition process. As shownin box 103, the surface of the conditioned contact layer may be cleaned,e.g., rinsed with a cleaning solution before an electrodepositionprocess to remove possible chemical residues and particles from thesurface of the contact layer.

It should be noted that the surface activation step is very importantbecause electrodeposition efficiency on a surface depends on the natureof that surface on which a material is deposited. An activated surfaceis a material surface that is electrochemically active and can beelectroplated with efficiency. If the surface is electrochemicallypassive, electrodeposition efficiency is generally low and adhesion ispoor. However, on an active, or activated, surface electrodepositionefficiency is higher and more consistent. Consistent electrodepositionefficiency yields consistent thickness for the electrodepositedmaterial. In the present invention CIGS type absorber layers are formedemploying precursor stacks such as Cu/Ga/In or Cu/Ga/Cu/In stacks. Thethicknesses of the layers within the stack need to be tightly controlledto be able to control the Cu/(In+Ga) and Ga/(In+Ga) molar ratios whichare typically below 1 and which are important for the quality of theresulting absorbers and the performance of solar cells fabricated onsuch absorbers. A typical target ratio for Cu/(In+Ga) may be in therange of 0.8-0.95. In a roll-to-roll system the contact layer on which afirst layer such as a Cu layer would be deposited may be exposed to theatmosphere for different periods of time depending on the location onthe roll. For example, in a roll that may be 5000 ft long, the contactlayer at the beginning of the roll may be coated with Cu within a fewminutes whereas a portion of the contact layer at the end of the rollmay be coated after 41 hours if the continuous flexible workpiece movesat a rate of 2 ft/minute. Such variation in exposure of the contactlayer to atmosphere may induce differences in the condition of thecontact layer surface due to oxidation, exposure to chemical fumes etc.Plating efficiency of the Cu layer on the contact layer may then bedifferent on portions of the contact layer at the beginning of the rolland at the end of the roll. Such differences in efficiency, in turn,cause differences in the thickness of the Cu layer throughout theflexible workpiece and thus cause a change in the Cu/(In+Ga) molarratio. As a result, process yields are reduced, and manufacturability ofhigh efficiency solar cells at high yields cannot be achieved. Byemploying an activation chamber and activation process step before theelectrodeposition of the first layer on the contact layer, consistenceof electrodeposition efficiency of the first layer on the contact layeris assured throughout the roll and yield for consistent Cu/(In+Ga) ratiois assured.

Conditioning process of the present invention results in anelectroplating efficiency of more than 90% when a subsequentelectroplating process is performed and the first metal layer such as acopper layer is electroplated onto the activated surface. For example,an activated surface formed on the contact layer by a cathodicconditioning process provides more than 90% electroplating efficiencyfor the subsequent electroplating process, such as copperelectroplating. However, if the surface is electrochemically passive,the electroplating efficiency is low, less than 90%, maybe even as lowas 20-50%.

Boxes 104 through 108 show a process sequence to form a precursor stackof the present invention. As shown in box 104, in a firstelectrodeposition step, a Group IB material such as copper, may beelectrodeposited on the conditioned and cleaned surface of the contactlayer. This step is followed by a cleaning step to clean the surface ofthe electrodeposited Group IB material, box 105. As shown in box 106, ina second electrodeposition step, a first Group IIIA material, such asgallium, may be electrodeposited on the surface of the cleaned Group IBmaterial layer. This step is followed by a cleaning step to clean thesurface of the electrodeposited first Group IIIA material layer, box107. As shown in box 108, in a third electrodeposition step, a secondGroup IIIA material, such as indium, may be electrodeposited on thesurface of the cleaned first Group IIIA material layer, which completesthe precursor stack. The precursor stack may be cleaned and driedfollowing step, box 109. The precursor stack may be reacted in presenceof Group VIA materials, such as selenium and sulfur with gas phasedelivery, to form an absorber, box 10.

Alternately, the precursor layer in box 108 may be just cleaned withoutdrying, as shown in box 111, to electrodeposit a Group VIA material ontothe precursor stack as shown in box 112. Following the electrodepositionprocess, the precursor stack with the Group VIA layer is cleaned, box113, and reacted to form an absorber, box 114. During the reaction,optionally, additional Group VIA materials may be introduced to theforming absorber.

The roll-to-roll processing approach of the present invention offersseveral advantages. Electrodeposition is a surface sensitive process.Defects in electrodeposited layers mostly originate from the surfacethey are plated on. Therefore, it is preferable to minimize handling ofsubstrates in an electroplating approach. Surfaces to be plated need tobe protected from physical contact, particles etc. that may later causedefectivity in the films deposited on such surfaces. Plating efficiencyand the thickness uniformity of electroplated layers are also affectedby the condition of the surface they are plated on. For example,electrodeposition of Cu, Ga or In on a chemically active, fresh surfaceis a much more repeatable process compared to electrodeposition on asurface that may be exposed to air, chemical vapors or, in general, tooutside environment for varying amounts of time. In a roll-to-rollprocess all depositions are done in a controlled environment (enclosurefor the roll not shown in figures) and the time between depositions areminimized unlike a batch process that requires several loading andunloading steps to deposit a stack of materials on a base. In thepresent roll-to-roll process a material, such as Cu is plated on asection of the base. The surface of this plated material is fresh andactive after plating and after the water rinse step. Therefore, whensection moves into the next plating bath, for example a Ga or In platingbath, within a few seconds or minutes, deposition initiates on thisactive surface. If the velocity of the foil base is constant, then theGa or In plating always operates on the same Cu surface in terms ofactivity. This provides highly repeatable results in terms of thicknessand uniformity of the In and Ga layers. Same is true for the Cu layeralso.

If the Cu layer is first to be deposited on the flexible foil base, thesurface of the flexible foil base may first be activated by passing itthrough a pre-deposition electrolyte and applying a pre-depositionprocess step or conditioning to the surface. The predeposition processstep may be an etching step or an electrotreating step such as acathodic conditioning step comprising applying a cathodic voltage to thebase with respect to an electrode in the pre-deposition electrolyte oran anodic conditioning step comprising applying an anodic voltage to thebase with respect to an electrode in the pre-deposition electrolyte.Conditioning step may also include a pickling step; or a deposition stepcomprising depositing a fresh layer on the base before the deposition ofCu. In all such cases, an active surface may be provided to the Cuelectrodeposition step so that this step yields repeatable results interms of Cu layer thickness and uniformity. As described before,thickness and uniformity control for deposited Cu, In and/or Ga layersare of utmost importance since Cu/(In+Ga) and Ga/(In+Ga) molar ratiosneed to be controlled throughout the base.

FIG. 3 shows an exemplary roll-to-roll electroplating system 30 withcapability to produce, on a flexible foil base 22, metallic stackscomprising Cu, In and Ga with excellent thickness control anduniformity. The electroplating system 30 comprises a series of processunits, a supply spool 20, a return spool 21 and a mechanism (not shown)to direct the flexible foil base 22 from the supply spool 20 to thereturn spool 21 through the series of process units. The series ofprocess units comprises at least one Cu electroplating unit 31, at leastone Ga electroplating unit 32, and at least one In electroplating unit33. It should be noted that the order of these electroplating units maybe changed to obtain various stacks on the base. For example, the orderof the electroplating units shown in FIG. 3 would yield a stack ofCu/Ga/In on the base. Changing this order and optionally adding otherelectroplating units one may obtain stacks such as Cu/In/Ga, In/Cu/Ga,Ga/Cu/In, Cu/Ga/Cu/In, Cu/Ga/Cu/In/Cu, Cu/In/Cu/Ga, Cu/In/Cu/Ga/Cu etc.It should be noted that many more iterations of such stacks arepossible. However, stacks initiating with a Cu layer are preferredbecause Cu plating yields highly controlled, good morphology coatings athigh plating efficiency, and Cu is a good base on which Ga and/or Infilms can be electroplated. In the following, the present invention willbe described using the configuration in FIG. 3 with the electroplatingsystem 30 comprising one of each of a Cu electroplating unit, a Gaelectroplating unit and an In electroplating unit.

In the electroplating system 30 of FIG. 3 there is, preferably, aconditioning unit 34 that conditions the surface of the flexible foilbase 22 on which a Cu layer will be deposited in the Cu electroplatingunit 31. A typical structure of the flexible foil base 22 is shown inFIG. 3A. The flexible foil base 22 comprises a flexible foil substrate45 and a conductive layer 46 or a contact layer deposited on a firstsurface 45A of the flexible foil substrate 45. The flexible foilsubstrate 45 may be made of any polymeric or metallic foil, butpreferably it is a metallic foil such as a 20-250 um thick stainlesssteel foil, Ti foil, Al foil or aluminum alloy foil. Various metallicfoil substrates (such as Cu, Ti, Mo, Ni, Al) have previously beenidentified for CIGS(S) solar cell applications (see for example, B. M.Basol et al., “Status of flexible CIS research at ISET”, NASA DocumentID: 19950014096, accession No: 95N-20512, available from NASA Center forAeroSpace Information). The conductive layer 46 may be in the form of asingle layer or alternately it may comprise a stack of various sublayers(not shown). Preferably, the conductive layer comprises at least onediffusion barrier layer that prevents diffusion of impurities from theflexible foil substrate 45 into the layers to be electrodeposited andinto the CIGS(S) layer during its formation. Materials of the conductivelayer 46 include but are not limited to Ti, Mo, Cr, Ta, W, Ru, Ir, Os,and nitrides and oxy-nitrides of these materials. Preferably, the freesurface 46A of the conductive layer 46 comprises at least one of Ru, Irand Os for better nucleation of the electroplated layers.

In this example, electrodeposition is carried out on the free surface46A of the conductive layer 46. The back surface 45B of the flexiblefoil substrate 45 may optionally be covered with a secondary layer 47(shown with dotted line) to protect the flexible foil substrate 45during annealing/reaction steps that will follow to form the CIGS(S)compound, or to avoid buckling of the flexible foil substrate 45. It isimportant that the material of the secondary layer 47 be stable inchemistries of the Cu, In and Ga plating baths, i.e. not dissolve intoand contaminate such baths, and also be resistant to reaction with GroupVIA elements. Materials that can be used in the secondary layer 47include but are not limited to Ru, Os, Ir, Ta, W etc. Use of a secondarylayer 47 comprising at least one of Ru, Ir and Os has an added benefit.Such materials are very resistant to reaction with Se, S and Te.Therefore, after any reaction step that forms CIGS(S) compound layer onthe free surface 46A of the conductive layer 46, the secondary layerprotects the flexible foil substrate 45 from reaction with Se, S or Teand leaves a surface that can be soldered easily. In prior art devicesMo was used as the secondary layer 47. During selenization and/orsulfidation processes or during the growth of the CIGS(S) absorber, thisMo layer reacted with Se and/or S forming a Mo(S,Se) surface layer.After solar cells are completed, they need to be interconnected to formmodules. Interconnection involves soldering or otherwise attaching backsurface of each cell to the front surface of the adjacent cell. AMo(S,Se) layer on the back of the cell cannot be soldered effectively,therefore physical removal of the selenized and/or sulfidized Mo surfaceis needed. However, a surface comprising at least one of Ru, Ir and Oscan be soldered easily without the added step of removing a selenized orsulfidized surface layer because these materials do not appreciablyselenize or sulfidize.

Referring back to FIG. 3, the flexible foil base 22 passes through aconditioning unit 34, and an optional cleaning unit 35, before enteringinto the Cu electroplating unit 31. In the conditioning unit 34, thesurface of the flexible foil base 22 (such as the free surface 46A ofthe conductive layer 46 in FIG. 3A) is conditioned to render it readyfor electrodeposition with Cu. Such conditioning may involve exposingthe free surface 46A to an acidic or basic solution for etching and/oractivation, applying a cathodic or anodic voltage to the free surface46A with respect to an electrode while both the electrode and the freesurface 46A are exposed to an electrolyte, electrodepositing a seedlayer on the free surface 46A, or simply rinsing and wetting the freesurface 46A before it moves into the Cu electroplating unit 31. If onlya rinsing process is carried out in the conditioning unit 34, therewould not be a need to the cleaning unit 35. Otherwise cleaning unit 35is needed to remove any residual chemicals left on both faces of theflexible foil base 22 before it moves into the Cu electroplating unit31. In the present invention, if a seed layer is electrodeposited on thefree surface 46A in the conditioning unit 34, this seed layer may be aCu layer that is 2-50 nm thick and it may be deposited from a bath thatyields defect free uniform layers. Complexed Cu electrolytes with highpH are especially suitable for this purpose. Use of seed layers andvarious chemistries for electroplating are disclosed in Applicant'sco-pending U.S. application Ser. No. 11/266,013 filed Nov. 2, 2005entitled “Technique and Apparatus for Depositing Layers ofSemiconductors For Solar Cell and Modular Fabrication”, and U.S.application Ser. No. 11/462,685 filed Aug. 4, 2004 entitled “Techniquefor Preparing Precursor Films and Compound Layers for Thin Film SolarCell Fabrication and Apparatus Corresponding Thereto”, entire contentsof these applications are incorporated herein by reference.

Once a section of the free surface 46A of the conductive layer 46 isconditioned and cleaned it moves into the Cu electroplating unit 31.Within the Cu electroplating unit 31, the free surface 46A (or thesurface of the seed layer if a seed layer has been deposited in theconditioning unit 34) is exposed to a Cu plating bath 36A which may becirculated between a first reservoir 36AA and a first chemical cabinet36A′. The Cu plating bath 36A may be filtered and replenished duringcirculation or while in the first chemical cabinet 36A′. Measurement andcontrol of various bath parameters, such as additive content, Cucontent, temperature, pH etc. may be continuously or periodicallycarried out within the first chemical cabinet 36A′ to assure stabilityof the Cu deposition process. Electrical connection to the conductivelayer 46 (or to the flexible foil substrate 45 if the foil substrateitself is conductive) may be achieved by various means including throughrollers 39 which may be touching the flexible foil base 22 at, at leastpart of its back or front surfaces. Preferably, front surface contactsare made at the two edges avoiding physical contact with most of thefront surface which may be damaged or contaminated by contacts. A firstanode 40A is placed in the Cu plating bath 36A and a potentialdifference is applied between the first anode 40A and the portion of theconductive layer 46 within the Cu electroplating unit 31, to deposit Cuon the portion of the free surface 46A that is exposed to the Cu platingbath 36A as the flexible foil base 22 is moved.

The portion of the flexible foil base 22 processed in the Cuelectroplating unit 31, passes through the Cu cleaning unit 37A andenters into the Ga electroplating unit 32. Within the Ga electroplatingunit, the surface of the already deposited Cu layer is exposed to a Gaplating bath 36B which may be circulated between a second reservoir 36BBand a second chemical cabinet 36B′. The Ga plating bath 36B may befiltered and replenished during circulation or while in the secondchemical cabinet 36B′. Measurement and control of various bathparameters, such as additive content, Ga content, temperature, pH etc.may be continuously or periodically carried out within the secondchemical cabinet 36B′ to assure stability of the Ga deposition process.Electrical connection to the conductive layer 46 (or to the flexiblefoil substrate 45 if the flexible foil substrate itself is conductive)may be achieved by various means including through rollers 39 which maybe touching the base at, at least part of its back or front surfaces.Preferably, front surface contacts are made at the two edges avoidingphysical contact with most of the front surface which may be damaged orcontaminated by contacts. A second anode 40B is placed in the Ga platingbath 36B and a potential difference is applied between the second anode40B and the portion of the conductive layer 46 within the Gaelectroplating unit 32, to deposit Ga on the portion of the Cu surfacethat is exposed to the Ga plating bath 36B as the flexible foil base 22is moved.

The portion of the flexible foil base processed in the Ga electroplatingunit 32, passes through the Ga cleaning unit 37B and enters into the Inelectroplating unit 33. Within the In electroplating unit, the surfaceof the already deposited Ga layer is exposed to an In plating bath 36Cwhich may be circulated between a third reservoir 36CC and a thirdchemical cabinet 36C′. The In plating bath 36C may be filtered andreplenished during circulation or while in the third chemical cabinet36C′. Measurement and control of various bath parameters, such asadditive content, In content, temperature, pH etc. may be continuouslyor periodically carried out within the third chemical cabinet 36C′ toassure stability of the In deposition process. Electrical connection tothe conductive layer 46 (or to the flexible foil substrate 45 if theflexible foil substrate itself is conductive) may be achieved by variousmeans including through rollers 39 which may be touching the flexiblefoil base at, at least part of its back or front surfaces. Preferably,front surface contacts are made at the two edges avoiding physicalcontact with most of the front surface which may be damaged orcontaminated by contacts. A third anode 40C is placed in the In platingbath 36C and a potential difference is applied between the third anode40C and the portion of the conductive layer 46 within the Inelectroplating unit 33, to deposit In on the portion of the Ga surfacethat is exposed to the In plating bath 36C as the base 22 is moved.After In electrodeposition the portion of the flexible foil basecomprising the all-electroplated Cu/Ga/In stack is passed through acleaning/drying unit 38 and moved to the return spool 21.

It should be noted that additional process units may be added to theelectroplating system 30 of FIG. 3. For example, another Cuelectroplating unit and another cleaning unit may be inserted betweenthe Ga cleaning unit 37B and In electroplating unit 33 to fabricate aCu/Ga/Cu/In stack. The anodes employed in the electroplating units maybe inert anodes or they may be dissolvable anodes of Cu, In and Ga forCu electrodeposition, In electrodeposition and Ga electrodeposition,respectively. The thicknesses of the Cu, In and Ga layers within thestack may range from 10 nm to 500 nm. Details of the cleaning orcleaning/drying units are not shown in FIG. 3. However, establishedcleaning means, such as spraying the cleaning solution onto the part tobe cleaned or immersing the part in the cleaning solution, may be usedin these units. Air knives directing high speed air or inert gas ontothe part to be dried may be used as the drying means. The drying gas maybe pre-filtered and warmed for effective and fast drying.

We so far described an example of a system and process for roll-to-rollelectrodeposition of stacks comprising Group IB and Group IIIAmaterials. Other processing units may be added to the electroplatingsystem of FIG. 3 to extend its functionality as will be described next.

FIG. 4 depicts a roll-to-roll processing system 50 comprising a GroupIB-IIIA electroplating unit 51 and a Group VIA material electroplatingunit 62. The Group IB-IIIA plating unit 51 electrodeposits the Group IBmaterials and Group IIIA materials on the flexible foil base 22 forminga metallic precursor film and may, for example, comprise all or most ofthe components of the electroplating system 30 of FIG. 3. As an example,the Group IB-IIIA plating unit 51 may deposit Cu, Ga and In layers andmay comprise the conditioning unit 34, the cleaning unit 35, the Cuelectroplating unit 31, the Cu cleaning unit 37A, the Ga electroplatingunit 32, the Ga cleaning unit 37B, and the In electroplating unit 33 ofFIG. 3. Instead of the cleaning/drying unit 38 of FIG. 3, anothercleaning unit (without drying) may be employed so that the flexible foilbase 22 coated or electrochemically coated with Cu, Ga and In moves intothe Group VIA material electroplating unit 62 with a clean and wetsurface. In the Group VIA material electroplating unit 62, a layer of atleast one of Se, S and Te, preferably Se, is deposited onto the metallicprecursor film. The flexible foil base with the “metallicprecursor/Group VIA material” stack may then be passed through a finalcleaning/drying module 63 and rolled onto the return spool 21. Presenceof a Group VIA material on the metallic precursor film comprising Cu, Inand Ga has advantages. One such advantage is the protection provided bythe Group VIA material to the surface of the metallic precursor film.Indium and Ga are soft, low melting materials and they are vulnerable toeasy scratching during rolling and handling. By depositing a Group VIAmaterial such as Se on the metallic precursor film this vulnerability isreduced or eliminated so that the flexible web may be rolled onto thereturn spool 21 safely. The thickness of the electroplated Group VIAmaterial may be in the range of 10-2000 nm.

The roll-to-roll processing system of FIG. 4 may accommodate an optionalannealing unit 64 as shown in FIG. 4. When used, the annealing unit 64will cause a reaction between the electrodeposited metallic precursorfilm and the electrodeposited Group VIA material and form a reactedprecursor layer on the flexible foil base 22. If the Group VIA materialis Se, the reacted precursor layer may comprise phases such as Cu, In,Ga, Cu—Ga, Cu—In, In—Ga, Cu—Se, In—Se, Ga—Se, Cu—In—Se, Cu—Ga—Se,In—Ga—Se and Cu—In—Ga—Se, depending on the temperature applied in theannealing unit 64 and the time spent in the annealing unit 64. Thetemperature applied by the annealing unit may be in the range of 100-550C, preferably in the range of 200-450 C. After exiting the annealingunit 64, the flexible web comprising the reacted precursor layer may berolled onto the return spool 21 safely. A packing sheet may also berolled along with it as described with reference to FIG. 2. It should benoted that the Group VIA material electroplating unit 62 may be similarto the electroplating units described with reference to FIG. 3. Theannealing unit 64 may be similar to a design described in co-pendingU.S. patent application Ser. No. 11/549,590 filed Oct. 13, 2006 entitled“Method and Apparatus For Converting Precursor Layers Into PhotovoltaicAbsorbers”, entire contents of which are incorporated herein byreference.

The examples above employed a flexible foil base 22 such as the onedepicted in FIG. 3A. In the flexible foil base 22 of FIG. 3A, theconductive layer 46 and the optional secondary layer 47 may be depositedon the flexible foil substrate 45 by various deposition techniques suchas evaporation, sputtering etc. in a separate system. It is, however,possible to integrate another electroplating or electroless platingmodule to the systems of FIGS. 3 and 4 so that the flexible foilsubstrate 45 gets electroplated with at least one of a conductive layeror contact layer and a secondary layer before it moves into otherprocess units such as the Group IB-IIIA electroplating unit of FIG. 4.This way defects in the electroplated Cu, In and Ga layers that are dueto defects in or on the contact layers (such as scratches, pinholes andother defects) are avoided since contact layers are freshly depositedand then get coated with Cu, Ga and In. The contact layer for thisapproach needs to comprise materials that can be electroplated orelectroless plated and at the same time be a good ohmic contact toCIGS(S) material and not react extensively with S and/or Se. Such layersare disclosed in Applicant's co-pending U.S. application Ser. No.11/266,013 filed Nov. 2, 2005 entitled “Technique and Apparatus forDepositing Layers of Semiconductors For Solar Cell and ModularFabrication”, and U.S. application Ser. No. 11/462,685 filed Aug. 4,2004 entitled “Technique for Preparing Precursor Films and CompoundLayers for Thin Film Solar Cell Fabrication and Apparatus CorrespondingThereto” and they comprise materials such as Ru, Ir and Os. It should benoted that by exposing the back side of the flexible foil substrate to acontact electroplating solution and deposition current, it is possibleto electroplate a secondary layer on the back side of the substrate asthe front face is plated by a contact layer.

In two-stage techniques, which involve deposition of a metallicprecursor film comprising Cu, In and Ga and then reaction of themetallic precursor film with at least one of Se and S, individualthicknesses of the Cu, In and Ga layers need to be well controlledbecause they determine the final stoichiometry or composition of thecompound layer after the reaction step. The roll-to-roll depositionapproach of the present invention lends itself well for smart processcontrol so that these thicknesses may be monitored and controlled usingin-situ measurement devices such as X-ray fluorescence (XRF). XRF probesmay be placed at various positions in the systems of FIG. 2, FIG. 3 andFIG. 4 and these probes may monitor the deposited thicknesses of Cu, In,Ga and optionally Se layers. If there is any discrepancy between thetarget and deposited thickness of any of the Cu, In, Ga layers, thepower supply controlling that thickness may be sent a signal by the XRFtool to increase or decrease the plating current density to keep thefilm thickness within a targeted window. Such approaches are describedin more detail in Applicant's co-pending U.S. Provisional ApplicationSer. No. 60/744,252 filed Apr. 4, 2006 entitled “Composition Control forPhotovoltaic Thin Film Manufacturing”.

Once the metallic precursor films, or the “metallic precursor/Group VIAmaterial” stacks, or the reacted precursor layers of the presentinvention are formmed, reaction or further reaction of these layers withGroup VIA materials may be achieved by various means. For example, theselayers may be exposed to Group VIA vapors at elevated temperatures.These techniques are well known in the field and they involve heatingthe layers to a temperature range of 350-600° C. in the presence of atleast one of Se vapors, S vapors, and Te vapors provided by sources suchas solid Se, solid S, solid Te, H₂Se gas, H₂S gas etc. for periodsranging from 5 minutes to 1 hour. In another embodiment a layer or multilayers of Group VIA materials may be deposited on the metallic precursorlayers and then heated up in a furnace or in a rapid thermal annealingfurnace and like. Group VIA materials may be evaporated on, sputtered onor plated on the metallic precursor layers in a separate process unit.Alternately inks comprising Group VIA nano particles may be prepared andthese inks may be deposited on the metallic precursor layers to form aGroup VIA material layer comprising Group VIA nano particles. Dipping,spraying, doctor-blading or ink writing techniques may be employed todeposit such layers. Reaction may be carried out at elevatedtemperatures for times ranging from 1 minute to 30 minutes dependingupon the temperature. As a result of reaction, the Group IBIIIAVIAcompound is formed. It should be noted that reaction chambers may alsobe added to the apparatus of FIG. 4 or the annealing unit 64 may be areaction unit to carry out the whole process in-line so that theflexible foil base with a fully formed CIGS(S) layer on its surface maybe rolled onto the return spool 21.

In the examples above, systems with horizontal web geometry have beendiscussed. It should be noted that the concepts of the present inventionmay be applied to systems where the flexible foil base travels in avertical position or at any angle with respect to the horizontal plane.Depositions may be carried out on the horizontal web in either “depositup” or “deposit down” manner. The flexible foil substrate may move fromleft to right or vice-versa. It may move continuously or in a stepwisemanner. It may also move with an oscillating “back-and-forth” motion. Itis possible to deposit some layers onto the flexible foil base as it ismoved in one direction and then deposit more layer(s) as the foil ismoved back in the reverse direction. DC, AC, pulsed or pulse-reversetype power supplies, among others, may be used for the electrodepositionsteps.

Solar cells may be fabricated on the Group IBIIIAVIA compound layers ofthe present invention using materials and methods well known in thefield. For example a thin (<0.1 microns) CdS layer may be deposited onthe surface of the compound layer using the chemical dip method. Atransparent window of ZnO may be deposited over the CdS layer usingMOCVD or sputtering techniques. A metallic finger pattern is optionallydeposited over the ZnO to complete the solar cell.

Although the present invention is described with respect to certainpreferred embodiments, modifications thereto will be apparent to thoseskilled in the art.

1. A system for forming an absorber structure for solar cells on a frontsurface of a continuous flexible workpiece as the continuous flexibleworkpiece is advanced through units of the system, comprising: aconditioning unit to condition the front surface of the continuousflexible workpiece to form activated surface portions, wherein theactivated surface portions present a consistently active surfacesubstantially along the continuous flexible workpiece forelectroplating; a first electroplating unit to form a first layer of aprecursor stack by electroplating a metal belonging to one of Group IBand Group IIIA over one of the activated surface portions of thecontinuous flexible workpiece as the continuous flexible workpiece isadvanced through the first electroplating station; a first cleaning unitto clean the first layer deposited in the first electroplating unit; asecond electroplating unit to form a second layer of the precursor stackby electroplating another metal belonging to the other of the Group IBand Group IIIA over the first layer as the continuous flexible workpieceis advanced through the second electroplating units and while the firstlayer is continued to be electroplated onto a following one of theactivated surface portions of the surface of the continuous flexibleworkpiece in the first electroplating unit, wherein the first layer isdifferent from the second layer; a second cleaning unit to clean thesecond layer deposited in the second electroplating unit; and a movingassembly to hold and linearly move the continuous flexible workpiecethrough the units of the system, wherein the moving assembly comprises afeed spool to unwrap and feed unprocessed portions of the continuousflexible workpiece into the system and a take-up spool to receiveprocessed portions and wrap them around.
 2. The system of claim 1further comprising a third electroplating unit to form a third layer byelectroplating a further metal belonging to one of the Group IB andGroup IIIA over the second layer to obtain the precursor stack as thecontinuous flexible foil is advanced through the first, second and thirdelectroplating stations and while the second layer is continued to beelectroplated in the second electroplating station on the first layerthat is electroplated on the following activated portion of the surfaceof the continuous flexible workpiece, and while the first layer iscontinued to be electroplated onto a further following one of theactivated surface portions of the surface of the continuous flexibleworkpiece in the first electroplating station, wherein the third layeris different from the first and second layers.
 3. The system of claim 2further comprising one of a cleaning-drying unit to clean and dry thethird layer and a third cleaning unit to clean the third layer depositedin the third electrodeposition unit.
 4. The system of claim 3 furthercomprising an annealing unit to react the first, second and thirdlayers.
 5. The system of claim 3 further comprising a fourthelectrodeposition unit to deposit a fourth layer of a Group VIA materialover the third layer.
 6. The system of claim 5, wherein the Group VImaterial includes one of Se, S and Te.
 7. The system of claim 5 furthercomprising a cleaning-drying unit to clean and dry the fourth layer andan annealing unit to react the first, second, third and fourth layers.8. The system of claim 1, wherein the conditioning unit comprises atleast one of: an electrotreating chamber having an electrotreatingsolution and electrode that can be anodically or cathodically polarizedwith respect to the front surface of the continuous flexible workpieceto form the activated portions, a deposition chamber to deposit a seedlayer over the front surface of the continuous flexible workpiece apickling chamber to treat the front surface of the continuous flexibleworkpiece to form the activated portions, and an etching chamber to etchthe front surface of the continuous flexible workpiece to form theactivated portions.
 9. The system of claim 1, wherein the Group IBmaterial includes Cu and wherein the first Group IIIA material includesone of Ga and In.
 10. The system of claim 1 further comprising a packingsupply spool to provide a continuous packing sheet to place onto theprocessed portions as the continuous flexible workpiece is wrappedaround the take-up spool.
 11. The system of claim 9 further comprising adeposition monitoring unit to monitor and control the thickness ofdeposited first, second and third layers.
 12. The system of claim 11wherein the deposition monitoring unit provides a feedback signals toeach of the first, second and third electrodeposition units to cause thethicknesses of deposited first, second and third layers to convergetoward a predetermined thickness for each of the first, second and thirdlayers.
 13. The system of claim 12 wherein the convergence is tomaintain target ratios of Cu/In+Ga and Ga/In+Ga.
 14. A process offorming a precursor stack on a frontside of a continuous flexibleworkpiece using a system including a moving assembly, wherein thefrontside includes a conductive layer, comprising: moving the continuousflexible workpiece into and sequentially through a conditioning unit, anactivated surface cleaning unit, a first electroplating unit, a firstcleaning unit, a second electroplating unit, a second cleaning unit, athird electroplating unit, and a cleaning-drying unit by feedingpreviously unrolled portions of the continuous flexible workpiece froman input end of the system; conditioning the surface of the conductivelayer in the conditioning unit to form an activated surface portion,wherein the activated surface portion presents a consistently uniformsurface along substantially an entirety of the continuous flexibleworkpiece for electroplating; cleaning the activated surface portion inthe activated surface cleaning unit; forming a precursor stack over theactivated surface portion after cleaning the activated surface portion,comprising: forming a first material layer over the activated surfaceportion by electrodepositing one of a Group IB material and a Group IIIAmaterial in the first electroplating unit; cleaning the first materiallayer in the first cleaning unit; forming a second material layer overthe first material layer by electrodepositing the other of the Group IBmaterial and the Group IIIA material in the second electroplating unit,wherein the second material layer is different from the first materiallayer; cleaning the second material layer in the second cleaning unit;forming a third material layer over the second material layer bydepositing another of the Group IB material and the Group IIIA materialin the third electroplating unit, wherein the third material layer isdifferent from the first and the second material layers; and cleaningand drying the precursor stack in the cleaning-drying unit; and takingup and wrapping processed portion of the continuous flexible workpieceat an output end of the system.
 15. The process of claim 14 furthercomprising: cleaning the precursor stack in a third cleaning unit priorto cleaning and drying; and forming a fourth material layer over theprecursor stack by depositing at least one Group VIA material from afourth deposition unit; and wherein the moving the continuous flexibleworkpiece into and sequentially through includes the third cleaning unitand the fourth deposition unit.
 16. The process of claim 15 furthercomprising reacting the precursor stack and the fourth layer in anannealing unit; and wherein the moving the continuous flexible workpieceinto and sequentially through includes the annealing unit.
 17. Theprocess of claim 16, wherein the at least one Group VI materialcomprises one of Se, S and Te.
 18. The process of claim 15, wherein theat least one Group VI material comprises one of Se, S and Te.
 19. Theprocess of claim 15, wherein the at least one Group VIA material isdeposited by one of dipping the precursor stack into an ink solutionincluding nano-particles of the at least one Group VIA material anddepositing by electrodeposition the at least one Group VIA material ontothe precursor stack.
 20. The process of claim 14 further comprisingreacting the precursor stack in an annealing unit with at least oneGroup VIA material after cleaning and drying; and wherein the moving thecontinuous flexible workpiece into and sequentially through includes theannealing unit.
 21. The process of claim 14 wherein conditioningcomprises one of electrotreating the conductive layer in a processsolution with respect to an electrode by applying one of cathodic andanodic polarization deposit a seed layer onto the front surface of thecontinuous flexible workpiece and pickling the front surface of thecontinuous flexible workpiece to form the activated portion.
 22. Theprocess claim 14 wherein the conditioning comprises depositing a seedlayer on the contact layer.
 23. The process of claim 14, wherein theGroup IB material includes Cu and wherein the Group IIIA materialincludes one of Ga and In.
 24. The process of claim 14 furthercomprising, forming a fifth material layer after forming the secondlayer, wherein the fifth layer and the first layer are the same, andforming a sixth layer after forming the third layer, wherein the sixthand the second layer are the same.